1. Field of the Invention
The present invention relates to a method of manufacturing a photomask and a method of manufacturing a semiconductor device using the photomask.
2. Description of the Related Art
The manufacturing process for semiconductor devices involves the patterning process of forming various patterns on a semiconductor wafer, the so-called lithography process. The lithography process uses photomasks.
In recent years, the dimensional accuracy required of photomasks has become increasingly severe. For example, the in-plane dimensional uniformity of the photomask is required to be 10 nm or less. In the manufacture of photomasks, there is a large number of items to decide whether they are defective or non-defective. Conventionally, photomasks that do not meet specifications in any one of the items have been considered to be defective, i.e., have been rejected.
FIG. 8 show typical items and specification values of specifications of a photomask (e.g., a half-tone phase-shifting mask). Even in FIG. 8, 11 items are listed. Conventionally, photomasks that exceed the specification value in at least one of the 11 items have been rejected. Although the accuracy of the photomask manufacturing techniques has been increased, the yield of non-defective products has decreased because the dimensional accuracy of the photomasks has become increasingly severe.
The specifications of photomasks are required in order to obtain a desired exposure latitude in exposing patterns onto a semiconductor wafer. The conventional specifications have been determined so that a desired exposure latitude will be obtained even if every item has been set to its critical specification value. With actual photomasks, however, it is very rare that every item is set to its critical specification value. In most of the photomasks, even if certain items are set to values in excess of their respective specification values, the other items are set to within their respective specification values with margin. Heretofore, such photomasks would have been rejected. We have found the fact that such rejected photomasks include photomasks that ensure the desired exposure latitude, i.e., photomasks that offer no problem in mass production of semiconductor devices. If, even in the presence of items that have been set to values in excess of their respective specification values, the other items have been set to within their respective specification values with margin, it is possible to obtain a desired exposure latitude as a whole. That is to say, if a decremental change in exposure latitude due to items that have exceeded specification values falls below an incremental change in exposure latitude due to items that conform to specification values, a desired exposure latitude will be obtained as a whole.
FIG. 9 shows the conventional measurements of a half-tone phase-shifting mask which was rejected. As shown in FIG. 9, in this mask, the deviation of the average value of the pattern dimension from its target value is 13 nm in excess of ±10 nm, the specification value, and the in-plane uniformity of the pattern dimension of the mask is 4 nm (3σ), which is sufficiently smaller than the specification value, 8 nm (3σ). When a wafer was actually exposed through this mask to measure the defocus latitude and the exposure latitude, the desired exposure latitude was obtained. Prior related applications by the assignee of this application include Japanese Patent Application No. 2000-260285 filed Aug. 30, 2000 and Japanese Patent Application No. 2001-159380 filed May 28, 2001. (U.S. patent application Ser. No. 09/940,578 filed Aug. 29, 2001.)
In Japanese Patent Application No. 2000-260285, variations in the dimensions of semiconductor device patterns on a photomask (mask pattern) and their average values are measured and the exposure latitude is calculated from the measured data. The photomask that meets the given exposure latitude is decided to be non-defective. In the case of a phase-shifting photomask, the transmission factor and phase difference of a phase shifting layer is further measured. The measured data is also used to calculate the exposure latitude. The photomask that meets the given exposure latitude is decided to be acceptable.
In contrast to Japanese Patent Application No. 2000-260285, Japanese Patent Application No. 2001-159380 is adapted to estimate exposure latitude from drawing position accuracy and measured data on defective regions and repaired regions after repair of defects as well, allowing more accurate acceptance/rejection decision. In addition, if the desired exposure latitude is not obtained from a photomask, a customer who uses the mask further decides whether it can be used in the lithography process from the point of view of device characteristics and device manufacture. That is, the acceptance/determination decision is made with process management conditions in mind.
Microstructuring and large scale integration of semiconductor devices are now in progress; accordingly, more accurate acceptance/rejection decision is required with photomasks as well.